Chemical change, direction

We now consider briefly the matter of electrode potentials. The familiar Nemst equation was at one time treated in terms of the solution pressure of the metal in the electrode, but it is better to consider directly the net chemical change accompanying the flow of 1 faraday (7 ), and to equate the electrical work to the free energy change. Thus, for the cell... 

The relationship between current flow and chemical reactions was estabUshed by Faraday who demonstrated that the amount of chemical change was directly proportional to the quantity of charge passed (//) and to the equivalent weight of the reacting material. 

Retrosynthetic. The direction of chemical change opposite to that of laboratory execution of a reaction (synthetic direction) the reverse-synthetic sense. 

Many cliciiiical reactions evolve or absorb heat. When applying energy balances (consenatioit law for energy) in tccluiical calculations the heat (enthalpy) of reaction is often indicated in mole units so that tliey can be directly applied to demonstrate its chemical change. To simplify the presentation that follows, examine the equation ... 

Direct Current (DC). This current is transmitted for industrial uses only in exceptional situations. The most common sources of direct current are storage batteries and industrial devices called rectifiers, in which alternating current is changed (rectified) to direct current, as is used in electrolytic cells for the manufacture of chlorine gas, magnesium, aluminum, and a few other chemicals. The direct current is flowing from the source through the user application and back to the source, in one direction. The motor is primarily used for speed control of selected equipment. 

An electrochemical cell is a device by means of which the enthalpy (or heat content) of a spontaneous chemical reaction is converted into electrical energy conversely, an electrolytic cell is a device in which electrical energy is used to bring about a chemical change with a consequent increase in the enthalpy of the system. Both types of cells are characterised by the fact that during their operation charge transfer takes place at one electrode in a direction that leads to the oxidation of either the electrode or of a species in solution, whilst the converse process of reduction occurs at the other electrode. 

Coulometric analysis is an application of Faraday s First Law of Electrolysis which may be expressed in the form that the extent of chemical reaction at an electrode is directly proportional to the quantity of electricity passing through the electrode. For each mole of chemical change at an electrode (96487 x n) coulombs are required i.e. the Faraday constant multiplied by the number of electrons involved in the electrode reaction. The weight of substance produced or consumed in an electrolysis involving Q coulombs is therefore given by the expression... 

Like physical equilibria, all chemical equilibria are dynamic equilibria, with the forward and reverse reactions occurring at the same rate. In Chapter 8, we considered several physical processes, including vaporizing and dissolving, that reach dynamic equilibrium. This chapter shows how to apply the same ideas to chemical changes. It also shows how to use thermodynamics to describe equilibria quantitatively, which puts enormous power into our hands—the power to control the And, we might add, to change the direction of a reaction and the yield of products,... 

Although thermodynamics can be used to predict the direction and extent of chemical change, it does not tell us how the reaction takes place or how fast. We have seen that some spontaneous reactions—such as the decomposition of benzene into carbon and hydrogen—do not seem to proceed at all, whereas other reactions—such as proton transfer reactions—reach equilibrium very rapidly. In this chapter, we examine the intimate details of how reactions proceed, what determines their rates, and how to control those rates. The study of the rates of chemical reactions is called chemical kinetics. When studying thermodynamics, we consider only the initial and final states of a chemical process (its origin and destination) and ignore what happens between them (the journey itself, with all its obstacles). In chemical kinetics, we are interested only in the journey—the changes that take place in the course of reactions. 

Most of the examples of chemical equations presented in Table 4.1 include an arrow ( ) symbol. This implies a direction to the chemical change represented this is a process of reactant(s) becoming produces). However, sometimes students meet reactions where an equality sign (=) or a double-headed arrow symbol is... 

To describe in fundamental terms the dissolution of coal in a hydrogen-donor solvent requires an experimental approach that allows the chemical changes that occur within the coal during dissolution to be discussed. This, in turn, requires a direct method of determining the structural features in coal before it is reacted. 

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